Coaxial electron capture detector with thermionic emission electron source

Abstract

An electron capture detector in which electrons for the reaction are supplied by a filament in a cylinder through which guard gas is passed. Apertures in the cylinder permit electrons from the filament to pass into an annular space surrounding the cylinder and between it and a collector. Sample gas is passed between the aperture and the collector, the pressures being such as to be capable of minimizing the diffusion of sample gas to the filament.

We claim: 
     
       1. A coaxial nonradioactive electron capture detector comprising: a hollow cylinder,   a filament mounted in said cylinder,   means defining an aperture in said cylinder in the vicinity of said filament,   metal located at least in the vicinity of said aperture so as to attract electrons from said filament to said aperture when properly biased with respect to the filament,   a collector of conductive material spaced from and lying outside of the cylinder in the vicinity of the aperture defined by said means,   means defining an input port to the interior of said cylinder at a point on one side of said filament,   means defining an exhaust port to the interior of said cylinder at a point on the other side of said filament, and   means for conducting gas to be analyzed, when present, through the space between said aperture and said collector.   
     
     
       2. A coaxial nonradioactive electron capture detector, comprising a hollow member of conducting material having an axis,   a filament mounted in said member at a given point along said axis,   means defining at least one aperture in said member at the same point along said axis as said filament,   a collector made of conductive material spaced from and outside of said member in the vicinity of apertures defined by said means,   an electrical connection to said collector,   means insulating said filament, said collector, and said hollow member from one another so that separate potentials can be applied to each,   means defining an input port to the interior of said hollow member at a pont in one direction along said axis from said filament,   means defining an exhaust port from the interior of said hollow member at a point in the opposite direction along said axis from said filament, and   means for conducting gas to be analyzed, when present, through the space between said aperture and said collector.   
     
     
       3. A detector as set forth in claim 2 wherein said collector completely surrounds said hollow member in the vicinity of said aperture. 
     
     
       4. A detector as set forth in claim 2 wherein the space between said hollow member and said collector is so small as to induce laminar flow of gases flowing through said space and wherein means are provided within said hollow member between the inlet port thereof and said filament to induce laminar flow in the vicinity of said aperture. 
     
     
       5. A coaxial nonradioactive electron capture detector, comprising a hollow member having an axis,   a thermionic filament mounted within said member,   means for heating said filament when said means is energized,   means defining an aperture in the wall of said member having a position along the axis of said member that is the same as the axial position of at least part of said filament,   a collector of conductive material spaced from and lying outside of the member in the vicinity of the aperture defined by said means,   means defining an input port communicating with the interior of said member at a point that is displaced in one axial direction from said filament,   means defining an exhaust port communicating with the interior of said member at a point that is displaced in the opposite direction from said filament whereby gas introduced at said input port means will flow through the interior of said hollow member and past said filament in a direction that is substantially parallel to the axis of said member, and   means for conducting gas to be analyzed, when present, into the space between said aperture and said collector in such manner that it flows past said aperture in a direction that is substantially parallel to the axis of said member whereby the static pressure of the gases on opposite sides of said aperture may be made equal.   
     
     
       6. In an electron capture detector, the combination of means defining a source chamber, at least a portion of which is made of conductive material,   a thermionic source mounted in said source chamber that emits electrons when energized,   a detection chamber at least a portion of which is made of conductive material,   aperture means providing communication between said source chamber and said detection chamber, whereby electrons emitted from said source may diffuse from said source chamber to said detection chamber,   the said filament, the conductive material of said source chamber and the conductive material of said detection chamber being insulated from each other so that different electric potentials may be applied to each,   means for guiding inert gas, when present under pressure, through said source chamber and past said aperture means,   means for guiding gas including a sample when provided under pressure, through said detection chamber and past said aperture means,   whereby control of the respective pressures of said gases applied to said gas guiding means can create a static pressure at the point where said aperture means communicates with said source chamber that is equal to the static pressure at the point where said aperture means communicates with said detection chamber and thereby reduce the amount of sample gas that can reach said thermionic source.   
     
     
       7. An electron capture detector, comprising a source chamber,   a thermionic filament mounted in said source chamber,   a detection chamber,   a collector contained in said detection chamber,   aperture means in said source chamber and said detection chamber providing for the passage of electrons from said source chamber to said detection chamber,   metallic means located in said source chamber and in the vicinity of said aperture for collecting electrons emitted by said filament when suitable bias voltages are applied to said metallic means and said filament,   means for guiding gas through said source chamber and past said aperture means, and   means for guiding gas to be analyzed through said detection chamber and past said aperture,   whereby the flow conditions of the gases may be controlled so as to reduce the flow of gas from either chamber to the other.   
     
     
       8. A detector as set forth in claim 7 having means for respectively applying voltages to said metallic means and to said filament so as to establish a first electrical field in said source chamber of such direction as to urge electrons emitted by said filament toward said aperture means, and   means for applying a voltage to said collector so as to provide a second electrical field in said detection chamber between said collector and said metallic means that permits electrons to flow from said aperture means to said collector, the strength of said second electrical field being such that the electrons flow through said aperture means and into said detection chamber largely by diffusion.